In a high frequency package in which a high-frequency semiconductor device that operates in a high frequency band such as a microwave band or a millimeter wave band is mounted, the high-frequency semiconductor device is usually mounted within a cavity that is airtightly and electrically shielded by a cover, a seal ring, and a ground conductor, in view of an environmental resistance and an operational stability.
However, resonance is generated in a frequency band that a size of the cavity determined by a member such as a cover is about ½ or an integer multiple of a free-space propagation wavelength, and thus an operation of the semiconductor device within the cavity and characteristics of transmission lines become unstable. Particularly in a high-frequency semiconductor device that operates in a millimeter wave band (30 gigahertz to 300 gigahertz), the size of the device approximates the size of a propagation wavelength corresponding to a signal frequency. Therefore, it becomes difficult to set the size of the cavity for accommodating the device to be less than ½ of the propagation wavelength corresponding to the signal frequency, and a high-order resonant mode is likely to be generated. Particularly in a millimeter wave radar that operates in a 76-GHz band, its free-space propagation wavelength becomes about 4 millimeters in this frequency band and the size of a cavity required for mounting a plurality of high frequency circuits with a size of 1 to 3-mm square becomes about 10 millimeters. Consequently, cavity resonance is likely to be generated.
Conventionally, to suppress the cavity resonance, the following configuration has been proposed. That is, this configuration includes an opening formed in a surface-layer ground conductor on a substrate within a cavity, an impedance transformer that is electrically connected via the opening to the cavity and has a length of about ¼ of in-substrate effective wavelength of a signal wave, a tip-short-circuited dielectric transmission line with a length of about ¼ of in-substrate effective wavelength of a signal wave, a coupling aperture formed at a part of an inner-layer ground conductor that the impedance transformer is connected to the dielectric transmission line, and a resistor formed in the coupling aperture. In this configuration, the absorption efficiency of electromagnetic waves is improved and thus the cavity resonance is reliably suppressed, thereby stably operates a semiconductor device and transmission lines (see, for example, Patent Document 1).
Patent Document 1: International Publication No. WO06/001389